While this is mostly true for BF2, if you look back at 1942 it wasn't really the case. Special Weapons added many new features but were limited to the new maps only (new weapons, vehicles that could create entirely different types of games like the transport plane). Rise to Rome, on the other hand, sucked dick and really didn't add much that wasn't a reskin of something else or a slight variation of something already available.

BF2: Special forces added grappling hook and crossbows for zip lines. It played vastly different to BF2 due to the new mobility of infantry. It had its own maps and felt almost like a different game.

There was also night maps where you could barely see anything without the assistance of night vision, flashlights or the few street lights.

Also, special forces had different flying physics which some people disliked. You could put a Mi-24 Hind into a angled spin and just have your gunner spray and kill people with 30mm cannon splash damage. You moved so fast in a circle that no one could hit you since there were no lock on anti-air weapons.